Light-emitting diodes (LEDs) are a commonly used light source in applications including lighting, signaling, signage, and displays. LEDs have several advantages over incandescent and fluorescent lamps, including high reliability, long lifetime, and high efficiency. A typical prior art LED package is shown in FIG. 1. The die is a piece of semiconductor material that actually produces the emitted light. The contact and bond wire carry electrical current to the die. The substrate provides a physical mounting surface for the die and helps conduct away the heat generated by the die. The substrate shown in FIG. 1 is cup-shaped, which helps collect light from the die and redirect it upwards. In some preferred cases, including the example in FIG. 1, the substrate also conducts electrical current. The die is surrounded by an encapsulant, typically a polymer but can be other materials such as glass, which is transparent to the light wavelengths of interest. The encapsulant protects the die from mechanical damage, moisture, and atmospheric exposure. It also increases light extraction efficiency from the die relative to a die in air. The semiconductor die is typically a high-optical-index material such as sapphire, SiC, or GaP. Light inside the high-index material can only escape when it is incident on the die surface at an angle of incidence θ<θC=arcsin[nsurround/ndie], where nsurround and ndie are the indices of refraction of the surround and die. Light extraction efficiency is improved when this critical angle is as large as possible. Encapsulants have a higher index than air, and therefore they increase light extraction. The encapsulant is generally chosen to have as high an index as possible, but selection is typically constrained by other requirements such as transparency, resistance to thermal and photochemical degradation, hardness, and ease of application and curing in a manufacturing process.
The semiconductor die in an LED typically produces only a narrow spectrum of light, which is perceived by the eye as a single color such as red, blue, green, amber, etc. However, for many applications, especially lighting, broadband white light is preferred. A common approach to providing white light with LEDs is to overlay the die with a phosphor which absorbs some or all of the LED light and emits light at lower wavelengths, thus providing a mixture of colors that the eye perceives as white. This combination of LED phosphor is commonly referred to as a “white LED.” White LEDs can be made from a number of LED-phosphor combinations, including blue LED+yellow phospor, blue LED+a combination of red and green phosphors, and UV LED+a combination of red, green, and blue phosphors. The various approaches and difficulties of different materials combinations are well known in the art, such as described by Regina Mueller-Mach et al. in “High-Power Phosphor-Converted Light-Emitting Diodes Based on III-Nitrides,” IEEE Journal On Selected Topics In Quantum Electronics, Vol. 8, No. 2, March/April 2002.
Performance and reliability of white LEDs also depend on the method for applying the phosphor as well as the materials. The phosphor typically comes in the form of fine powder. The most commonly used method is to disperse this powder in a polymer binder and dispense the mixture directly onto the LED. The polymer binder can be the same material used for the encapsulant, and in fact the phosphor-binder mixture becomes functionally part of the encapsulant. FIGS. 2A and 2B show two examples. In previously developed white LEDs, as shown in FIG. 2A, the phosphor-containing volume was usually much larger than the die, which in turn caused the effective light source area to be much larger than the die. It is well-known in the art of optical design that in a high-collection optical systems the output beam has an etendue Eout=nout2AoutΩout, where nout is the index of the medium in which the beam emerges (typically air, with n≅1), Aout is the area of the beam, and Ωout is the solid angle of the beam. For high collection efficiency, it is accepted that Eout must be greater than or equal to the etendue of the light source itself. The phosphor layer is substantially Lambertian ±90°, which means that the etendue of the phosphor layer is approximately nbinder2×Aphosphor×π where nbinder is the optical index of the medium in which the phosphor is immersed. Since nbinder is typically equal to or at least similar to nencapsulant, and Aphosphor is larger than the area of the die, the etendue of the phosphor layer is clearly much larger than the etendue of the light emerging from the surface of the die into the encapsulant. This larger light source is unfavorable for many applications, especially those requiring that the light be collected and redirected into a highly collimated beam of light within a limited output aperture, for example flashlights, spotlights, and automotive headlight high beams.
Recent improvements have produced white LEDs in which the phosphor-binder mixture is limited to a small region directly on the die surface, largely co-extensive with the die. An example is shown in FIG. 2B. Note that this method of applying the phosphor is much easier when the die is a “flip chip” with all its contacts on the bottom surface, and therefore no bond wire on top. However, applying the phospor directly to the surface of the die has several disadvantages. The phosphor/binder mixture is subjected to the high temperatures of the die, which produces large amounts of heat and is typically the highest-temperature point in the package. The resulting high temperature causes color shifts due to the temperature sensitivity of the phosphor light emission. High temperatures also subject both binder and phosphor to temperature related degradation, which decreases light output over time. White LEDs accoding to FIG. 2 are also subject to color non-uniformity, as light emitted directly by the LED has a different spatial or angular pattern from light emitted by the phosphor.